To fulfil current emission legislations, most vehicles with internal combustion engines are today provided with an exhaust gas aftertreatment device comprising a catalytic converter arranged in an exhaust duct for reduction of exhaust gas emissions. The catalytic converter generally consists of a tubular, longitudinal body with an inlet and an outlet, where the inlet is in fluid communication with the outlet. The catalytic converter usually comprises at least one catalytic converter substrate in a channelled structure arranged between the inlet and outlet. Exhaust gases can pass through the channelled structure while being exposed to a large surface area of the catalytic converter. The inlet of the catalytic converter receives exhaust gas emissions from the internal combustion engine and the outlet is arranged for discharging exhaust gas emissions treated in the catalytic converter.
The most frequently used catalytic converter structure for petrol engines is generally Three Way Catalyst (TWC). For diesel engines generally Diesel Oxidation Catalyst (DOC) and Lean NOx Trap (LNT) are most frequently used. It is also common that the above mentioned catalytic converter structures are supplemented by a catalytic nitrogen oxide (NOx) converter, such as a substrate with selective catalytic reduction (SCR) functionality for improved NOx reduction. For substrates with SCR functionality, a liquid or gaseous reductant is introduced in the gas emission flow upstream the SCR substrate so that the reductant can mix with the exhaust gas emissions before the SCR substrate.
When using liquid reductant, such as a urea solution (“AdBlue”), the liquid reductant is sprayed into the exhaust duct upstream the catalytic NOx converter so that the liquid reductant can evaporate and mix with the exhaust gases before entering the catalytic NOx converter. Heat transferred from the exhaust gases is used for evaporating the liquid reductant. Sufficient mixture of the reductant is important for optimal reduction of NOx in the SCR substrate and for the SCR substrate to work properly.
US2007/0036694 discloses an example of an exhaust aftertreatment system where an injector is injecting a liquid reductant in the form of urea water into an exhaust duct and where a NOx catalyst is disposed downstream the injector. The liquid reductant is in this example injected onto inclined perforated plates arranged in the exhaust duct so that droplets of the injected liquid reductant impinge against the plates before reaching a wall surface of the exhaust duct. According to US2007/0036694, deposition of the liquid reductant on the wall surface is prevented and the liquid reductant is uniformly dispersed into the exhaust gas.
Conventional SCR exhaust gas aftertreatment systems commonly work well during normal operation conditions when the exhaust gas temperature is relatively high. However, a disadvantage of conventional SCR exhaust gas aftertreatment systems is that the liquid reductant is not evaporated sufficiently at lower temperature levels, i.e. typically in low-load situations. This might lead to wetting of the walls in the exhaust duct and reduction deposit formation on the walls and other components of the exhaust duct.
There is thus a need for an improved SCR exhaust gas aftertreatment device adapted to low temperature conditions.